1. Field of the Invention
This invention relates in general to the art of the cable transmission and distribution of broadband radio frequency signals and more particularly to improved radio frequency chokes for use in the circuits of various transmission and distribution equipment for separating and combining broadband radio frequency signals and single phase AC power signal which are simultaneously transmitted in the same cable.
2. Description of the Prior Art
In the art of the cable transmission and distribution of radio frequency (RF) signals, such as television signals, and the like, it is a common practice to simultaneously transmit a broadband of RF signals and a single phase AC power signal in the same coaxial cable with such simultaneous transmission being commonly referred to as frequency diplexing.
In a typical cable system, the RF signals originate or are transmitted from a central location known as the "headend". The coaxial cables used to carry the RF signals inherently have loss characteristics and amplifier stations are installed at appropriate locations along the cable to compensate for the losses and return the signal levels as closely as possible to what they were at the headend. The single phase AC power signal, which in most instances in the United States is 60 Hz, is needed to operate the amplifier stations and the AC power signal is introduced, or coupled, into the coaxial cable at appropriate locations and will travel along the cable with the RF signals. The single phase AC power signal is at a power level which is approximately 50,000 times greater than that of the RF signals, and must use different and separate circuitry to accomplish their totally different functions. Therefore, the single phase AC power signal must be separated from the RF signals within each amplifier station.
Other equipment is also used, in addition to the amplifier stations, in cable systems for distribution of the RF signals according to subscriber requirements. This other equipment is passive, i.e., it does not need the single phase AC power signal for operation. However, the passive equipment must be able to pass and distribute the AC power signal without interfering with the various operations that relate to the RF signals.
In the amplifier stations which must use the single phase AC power signal and in the passive equipment which must pass or distribute the AC power signals, special circuits are employed for separating the RF signals from the AC power signal. Also, the equipment used in the cable system for introducing, or coupling, the single phase AC power signal into the system employs special circuits, similar to the separating circuits mentioned above, which operate to combine the AC power signal and the RF signals. The special circuits will hereinafter be collectively referred to as the frequency diplexing circuits for clarity of this description.
One of the main problems with cable systems results from cascading of the many similar circuits used in the equipment provided along the length of the cable system. Each piece of the various types of equipment will have a characteristic frequency response, and it is desirable that each piece of equipment be capable of maintaining the relative level of all of the RF signals to each other. That is, the relative levels of the RF signals at the output end of each piece of the equipment should ideally be identical with the relative levels at the input end. This is often referred to as a "flat" response and means that the equipment is not contributing unwanted variations in signal levels regardless of the frequency of the signal. In actuality, circuits do not have perfectly "flat" frequency responses and degradation of the "flat " response becomes a bigger problem at higher frequencies and as the bandwidth of frequencies increases. In equipment of the same make, flatness degradations are usually of the same type and occur at about the same spot in the RF bandwidth. When the equipment is cascaded, the flatness degradations are cumulative and cause what is called a "signature". If the cascade is long and the flatness degradation of a single unit is large enough, the end-of-the-line flatness degradation will be unacceptably high causing severe deterioration of signal quality. Therefore, one of the objectives in equipment design is to keep flatness degradations to a minimum.
The frequency diplexing circuits used in the hereinbefore described cable equipment are by function and necessity in the main RF signal path of the cable system, and radio frequency (RF) chokes are the primary components in these frequency diplexing circuits because all the single phase AC current passes through them and they are connected directly to the RF signal path.
As is well known, an RF choke is an inductor which exhibits a high reactance or impedance to signals in the RF frequency range and low impedance to signals of lower frequency. In cable systems today, the frequency range of RF signals is from about 5 to 450 MHz. The RF chokes employed as described above in the cable transmission and distribution systems presents a high impedance to those frequencies of the RF signals, and offers virtually no impedance to the lower frequency single phase AC power circuit. This inherent characteristic of RF chokes makes them useful in the separation and combining of RF signals and single phase AC power signals. For example, if such an RF choke were connected with one end tied to the main line carrying both RF signals and AC power signals, and the other end tied to an AC input of a power supply, its function, ideally, would be to provide a low impedance path for the AC power signals to the power supply while presenting a very large impedance to the high frequency radio frequency signals. The result would be that the AC power signal is diverted to the power supply while the RF signals would continue completely unaffected. It should be stated that the separating function described above can only be fully effected by the RF choke in conjuncton with other components of the circuit.
Traditional RF chokes, unfortunately, do not offer a uniformly high impedance to all frequencies in the bandwidth of RF signals from 5 to 450 MHz. As is known, most so called traditional RF chokes consists of several turns of insulated wire wound around a ferromagnetic core. In cable systems, in order to maintain a sufficiently high inductive reactance, or impedance at the 5 MHz end of the frequency band, the RF chokes must have a relatively large number of turns of wire. Due to the physical configuration of these RF chokes, parasitic capacitances exist between the windings of the coil. These capacitances in conjunction with the inductance of the coil form parasitic resonances, most of which are series resonances. The presence of series resonances, along portions of the RF choke, cause significant reductions in its impedance at the resonant frequencies. The Q of these resonant circuits is high enough to cause significant and oftentimes sharp degradations in equipment flatness, and the RF signals are undesirably effected thereby.
Traditional chokes of the type described briefly above can perform well in cable systems having an upper frequency limit of approximately 220 MHz. However, increasing usage of such cable systems results in the need for wider bandwidths and these traditional RF chokes simply do not perform well at higher frequencies.
In addition to the effects on frequency response, the RF chokes used in the equipment of the cable system must be capable of passing several amperes of AC current. The wire used for the coil must, therefore, be large enough to carry relatively high currents, usually up to about 14 amperes in such cable transmission systems, without becoming excessively warm. Unfortunately, the larger the wire size the more troublesome is the parasitic resonance problem. High currents also pose problems in that core materials are likely to approach saturation thereby presenting the RF signals with an impedance which varies at the frequency rate of the single phase AC power signal. The effect of this is the unwanted modulation of RF signals and this problem is commonly referred to as "hum mod".
The above described problems due to high AC current can be effectively reduced by careful selection of wire size, core material, and core geometry. Unfortunately, solutions to these problems aggravate the parasitic resonance problem. Techniques have evolved to minimize the effects of these parasitics and are generally effective for use in cable systems which carry RF signals in the range of about 5 to 450 MHz. All of these techniques consist for the most part in reducing the Q of the parasitic resonances thereby turning sudden, sharp, and deep impedance variations into slow, smooth, and shallow variations. This is sometimes referred to in the art as "swamping".
Effective broadband suppression of parasitic resonances seem to be possible only through some sort of "swamping" technique, and as might be suspected, something must be sacrificed to obtain a flat response in this way. What swamping does, essentially, is increase the impedance of the choke at those frequencies where parasitic resonances occur and decrease the impedance of the choke at all other frequencies. The objective of good swamping design is to maximize the former and minimize the latter, and in such a way as to render an overall uniform impedance across the entire bandwidth of frequencies.
As mentioned earlier, there are techniques that have been used to give good performance in the frequency range of about 5 to 450 MHz. U.S. Pat. No. 4,394,631, for example, discloses a special RF choke which has proven to work well in this bandwidth. Briefly, this special prior art RF choke is disclosed as having a current carrying capacity of 10-12 amps with peak current as high as 15 amps, and is provided with a ferrite core having a diameter of 0.250 inches with a permeability suitable for the transmission of RF signals in the range of about 5 to 400 MHz. An 18 gage wire is wound around the core to provide a coil having 21 turns and a first RF resistor, in the form of a bead, is concentrically mounted on the seventh turn from one end of the coil. A second, or shunt resistor is connected in parallel between the opposite end of the coil and the seventh turn from the opposite end of the coil. The addition of the first resistor, i.e., the bead, adds a series impedance to the inductance of the RF choke. This, in conjunction with placement of the shunt resistor, results in a relatively efficient dampening, or swamping, of the parasitic series resonances in the RF choke so that it presents comparatively smooth variations in the impedance to the RF signals in the frequency range of about 5 to 450 MHz. However, above 450 MHz the uniformity of the impedance has been found to degrade significantly.
Current demands on cable system capabilities are continually increasing and the need for extended bandwidths and upper frequency limits beyond 550 MHz is imminent. Therefore, a need exists for a new and improved RF choke which overcomes some of the problems and shortcomings of the prior art.